1. Field of the Invention
The present invention relates to a photoelectric position measuring encoder including a scale formed of an electrically non-conductive material and having a graduation, a support member to which the scale is secured, a scanner for photoelectrically scanning the graduation and displaceable relative to the scale in a measuring direction for generating a position-dependent signal, and a slide or roller element for supporting the scanner on the scale surface.
2. Description of the Prior Art
Encoders described above are used for determining an incremental or absolute position in both machine-tools and in coordinate measuring machines.
In photoelectric position measuring encoders, the scale is scanned, without being contacted, by using a light source and photosensors. The signals generated by such scale scanning are relatively small and, therefore, susceptible to distortions.
German Patent No. 2,505,587 describes a photoelectric linear encoder to the improvement of which the present invention is directed. In the encoder of German Patent No. 2,505,587, a glass scale is secured to the housing with a highly elastic adhesive layer. A slide block supports the scanner on the glass scale, with the scale forming a guide for the scanner.
A drawback of this encoder consists in that the outer scale surface accumulates an electrical charge, which can become so large that a discrete electrical discharge via the scanner takes place. This discrete discharge generates an electrical pulse which acts as a distortion pulse. This distortion pulse is interpreted by the evaluation system in particular in incremental measuring systems, as a pulse which has to be counted or as a reference mark pulse.
U.S. Pat. No. 5,030,825 discloses an angular encoder in which a plurality of photosensors are used for scanning the scale. A scanning plate is provided between the scale and the photosensors. This scanning plate is formed of an electrically conductive material and is connected to a reference potential (to the ground). In this way, the scanning plate screens the photosensors from distortion electromagnetic fields.
The drawback of using such a scanning plate consists in that this plate cannot be formed of a transparent material on which very fine structured marks, on which are subjected to scanning, can be put. Moreover, distortion signals, which are outputted by the scale, cannot be eliminated by this scanning plate.
Accordingly, an object of the present invention is a photoelectric position measuring encoder with an improved distortion protection.